U.S. patent application number 17/043989 was filed with the patent office on 2021-02-04 for method for automatically calibrating a camshaft sensor in order to correct a gap jump.
The applicant listed for this patent is Continental Automotive France, Continental Automotive GmbH. Invention is credited to Denis Bouscaren.
Application Number | 20210033428 17/043989 |
Document ID | / |
Family ID | 1000005177304 |
Filed Date | 2021-02-04 |
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United States Patent
Application |
20210033428 |
Kind Code |
A1 |
Bouscaren; Denis |
February 4, 2021 |
METHOD FOR AUTOMATICALLY CALIBRATING A CAMSHAFT SENSOR IN ORDER TO
CORRECT A GAP JUMP
Abstract
A method for automatic calibration of a camshaft sensor for a
motor vehicle. The sensor includes a processing module configured
to generate, from a raw signal indicative of the variations in a
magnetic field which are caused by the rotation of a toothed target
and measured by a cell, an output signal indicative of the moments
at which the teeth pass past the cell. The calibration method makes
it possible, for each tooth, to determine a switching threshold not
only as a function of a local minimum and of a local maximum for
the tooth during the preceding revolution of the target, but also
as a function of a corrective value calculated from a local maximum
and/or a local minimum of the raw signal during the passage of a
preceding tooth past the cell during a new revolution.
Inventors: |
Bouscaren; Denis; (Toulouse,
FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Continental Automotive France
Continental Automotive GmbH |
Toulouse
Hannover |
|
FR
DE |
|
|
Family ID: |
1000005177304 |
Appl. No.: |
17/043989 |
Filed: |
March 28, 2019 |
PCT Filed: |
March 28, 2019 |
PCT NO: |
PCT/FR2019/050721 |
371 Date: |
September 30, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01D 5/24452 20130101;
G01D 5/2448 20130101; F01L 2820/041 20130101; F01L 1/047
20130101 |
International
Class: |
G01D 5/244 20060101
G01D005/244; F01L 1/047 20060101 F01L001/047 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 5, 2018 |
FR |
1852953 |
Claims
1. A method for automatic calibration of a camshaft sensor for a
motor vehicle engine, said sensor comprising: a toothed target
comprising at least two teeth (D1, D2, D3), a measurement cell
configured to supply a raw signal indicative of variations in a
magnetic field which are induced by a rotation (R) of the target,
and a processing module configured to supply, from the raw signal,
an output signal indicative of moments at which the teeth (D1, D2,
D3) of the target pass past the cell, said method comprising, for
each new revolution (N) of the target and for each tooth (D.sub.1):
determining a local minimum (m.sub.j,n) of the raw signal as a
space preceding said tooth (D.sub.j) passes past the cell,
determining a local maximum (M.sub.j,N) of the raw signal as said
tooth (D.sub.j) passes past the cell, calculating a switching
threshold (S.sub.j,N) for generating the output signal as a
function of a local minimum (m.sub.j,N-1) and of a local maximum
(M.sub.j,N) which are determined for said tooth (D.sub.j) in a
preceding revolution (N-1) of the target, wherein the switching
threshold (S.sub.j,N) is also calculated as a function of a
corrective value (.DELTA..sub.k,N) calculated as a function of a
local maximum (M.sub.k,N) of the raw signal determined during the
passage of a preceding tooth (D.sub.k) past the cell during the new
revolution (N), and of a local maximum (M.sub.k,N-1) of the raw
signal determined during the passage of said preceding tooth
(D.sub.k) past the cell during a preceding revolution (N-1).
2. The method as claimed in claim 1, wherein said corrective value
(.DELTA..sub.k,N) corresponds to a difference between the local
maximum (M.sub.k,N-1) determined for said preceding tooth (D.sub.k)
in the preceding revolution (N-1) and the local maximum (M.sub.k,N)
determined for said preceding tooth (D.sub.k) in the new revolution
(N).
3. The method as claimed in claim 1, wherein said corrective value
(.DELTA..sub.k,N) corresponds to a difference between an amplitude
(A.sub.k,N-1) of the raw signal for said preceding tooth (D.sub.k)
in the preceding revolution (N-1) and an amplitude (A.sub.k,N) of
the raw signal for said preceding tooth (D.sub.k) in the new
revolution (N).
4. The method as claimed in claim 2, wherein said switching
threshold (S.sub.j,N) is calculated as a function of said
corrective value (.DELTA..sub.k,N) only if the corrective value
(.DELTA..sub.k,N) is above or equal to a predetermined correction
threshold (.DELTA.S).
5. The method as claimed in claim 4, wherein a correction threshold
is defined for each tooth (D1, D2, D3) and for each revolution of
the target, and a correction threshold (.DELTA.S.sub.k,N) for said
preceding tooth (D.sub.k) in the new revolution (N) of the target
is defined by:
.DELTA.S.sub.k,N=(1-K).times.(M.sub.k,N-1-m.sub.k,N-1) where:
M.sub.k,N-1 corresponds to the value of the local maximum
(M.sub.k,N-1) determined for said preceding tooth (D.sub.k) in the
preceding revolution (N-1), m.sub.k,N-1 corresponds to the value of
the local minimum (m.sub.k,N-1) for said preceding tooth (D.sub.k)
in the preceding revolution (N-1), K is a predefined factor
comprised between 0 and 1.
6. The method as claimed in claim 5, wherein said switching
threshold (S.sub.j,N) is calculated according to:
S.sub.j,N=K.times.(M.sub.j,N-1-m.sub.j,N-1-.DELTA..sub.k,N)+m.sub.j,N-1
where: M.sub.j,N-1 corresponds to the value of the local maximum
(M.sub.j,N-1) determined for said tooth (D.sub.j) in the preceding
revolution (N-1), m.sub.k,N-1 corresponds to the value of the local
minimum (m.sub.k,N-1) determined for said tooth (D.sub.j) in the
preceding revolution (N-1), .DELTA..sub.k,N corresponds to the
corrective value (.DELTA..sub.k,N) calculated for said preceding
tooth (D.sub.k) in the new revolution (N).
7. The method as claimed in claim 6, wherein K is comprised between
0.7 and 0.8.
8. A camshaft sensor for a motor vehicle engine, comprising: a
toothed target comprising at least two teeth (D1, D2, D3), a
measurement cell configured to supply a raw signal indicative of
the variations in a magnetic field which are induced by rotation of
the target, and a processing module configured to supply, from the
raw signal, an output signal indicative of moments at which the
teeth (D1, D1, D3) of the target pass past the cell, said
processing module being configured, for each new revolution (N) of
the target and for each tooth (D.sub.j), to: determine a local
minimum (m.sub.j,N) of the raw signal as a space preceding said
tooth (D.sub.j) passes past the cell, determine a local maximum
(M.sub.j,N) of the raw signal as the tooth (D.sub.j) passes past
the cell, calculate a switching threshold (S.sub.j,N) for
generating the output signal as a function of a local minimum
(m.sub.j,N-1) and of a local maximum (M.sub.j,N-1) which are
determined for said tooth (D.sub.j) in a preceding revolution (N-1)
of the target, wherein said processing module is also configured to
calculate the switching threshold (S.sub.j,N) as a function of a
corrective value (.DELTA..sub.k,N) calculated as a function of a
local maximum (M.sub.k,N) of the raw signal during the passage of a
preceding tooth (D.sub.k) past the cell during the new revolution
(N), and of a local maximum (M.sub.k,N-1) of the raw signal during
the passage of said preceding tooth (D.sub.k) past the cell during
a preceding revolution (N-1).
9. The sensor as claimed in claim 8, wherein said corrective value
(.DELTA..sub.k,N) corresponds to a difference between a local
maximum (M.sub.k,N-1) determined for said preceding tooth (D.sub.k)
in the preceding revolution (N-1) and a local maximum (M.sub.k,N)
determined for said preceding tooth (D.sub.k) in the new revolution
(N).
10. A motor vehicle comprising a camshaft sensor claimed in claim
8.
11. The method as claimed in claim 3, wherein said switching
threshold is calculated as a function of said corrective value only
if the corrective value is above or equal to a predetermined
correction threshold.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the U.S. National Phase Application of
PCT International Application No. PCT/FR2019/050721, filed Mar. 28,
2019, which claims priority to French Patent Application No.
1852953, filed Apr. 5, 2018, the contents of such applications
being incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of sensors for
motor vehicles. In particular, the invention relates to a method
for automatically calibrating a motor vehicle camshaft sensor.
BACKGROUND OF THE INVENTION
[0003] A camshaft sensor is, for example, used in a motor vehicle
to determine which stroke of the combustion cycle is taking place
in a cylinder of the engine (admission stroke, compression stroke,
combustion stroke, or exhaust stroke). Such information for example
allows a computer to determine at what moment and into what
cylinder fuel is to be injected.
[0004] A camshaft sensor generally comprises a target (for example
a metal disk, the periphery of which is toothed), a magnetic-field
generator (for example a permanent magnet), a magnetic-field
measurement cell (for example a Hall-effect cell or a
magneto-resistive cell) and an electronic signal-processing
module.
[0005] The teeth of the target are generally all of the same
height, but may have spacings (spaces) and lengths that are not all
identical, so as to code the angular position of the target.
[0006] Thus, the rotation of the target and the passing of the
various teeth past the magnetic-field generator will generate
variations in the magnetic field measured by the measurement cell,
which variations can be analyzed in order to recognize the various
teeth of the target and to decode the angular position of the
target and, ultimately, the angular position of the camshaft
rigidly connected to the target.
[0007] The measurement cell supplies the processing module with a
raw signal indicative of the intensity of the magnetic field
measured. The processing module then generates, from this raw
signal, an output signal indicative of the moment at which the
various teeth of the target pass past the measurement cell.
[0008] This output signal is, for example, an electrical signal
comprising a succession of square waveforms. The high part of each
square form corresponds to the passage of a tooth past the
measurement cell. The low part of each square form corresponds to
the passage of a space past the measurement cell. Each high part of
a square form comprises a rising front and a falling front
corresponding more or less to the passing of the mechanical fronts
of the tooth past the measurement cell.
[0009] In general, each rising and falling front of the output
signal (namely each transition of the electrical signal) is
determined from a switching threshold that is predefined for the
raw signal. In other words, the output signal exhibits a rising
front when the raw signal passes above the switching threshold, and
the output signal exhibits a falling front when the raw signal
passes below the switching threshold. Conventionally, a switching
threshold corresponding to approximately 75% of the amplitude of
the raw signal is used (what is meant by the "amplitude of the raw
signal" is the difference between a maximum value and a minimum
value observed for said raw signal).
[0010] It is possible, for example, to define a fixed switching
threshold which does not change value during operation of the
sensor. However, such a solution is particularly imprecise insofar
as the minimum and maximum values of the raw signal may change
significantly during the operation of the sensor, notably as a
function of temperature.
[0011] It is therefore known practice in the prior art to update
the value of the switching threshold for each new revolution of the
target, as a function of the minimum and maximum values of the raw
signal which are observed during said revolution of the target. The
updated value of the switching threshold is then used for the next
revolution of the target. Such a solution improves the precision of
the sensor.
[0012] However, the precision of the sensor is generally also
impacted by deficiencies in the geometry of the target (for example
if the teeth do not all have exactly the same height). The
consequence of such deficiencies is that the magnitude of the gap
between the measurement cell and a tooth of the target is not the
same for each tooth. The raw signal then adopts different maximum
and minimum values for each tooth, and a switching threshold that
is defined as optimal for one of the teeth may prove to be entirely
inappropriate for another tooth.
[0013] It is therefore known practice in the prior art to determine
a different switching threshold for each tooth of the target. Each
switching threshold for each tooth of the target may be updated for
each new revolution of the target in order to be used for the next
revolution of the target. Such a solution improves the precision of
the sensor still further.
[0014] Nevertheless, the various solutions of the prior art are not
always able to achieve the precision required by certain motor
vehicle manufacturers for a camshaft sensor.
SUMMARY OF THE INVENTION
[0015] It is an aim of the present invention to overcome all or
some of the disadvantages of the prior art, notably those set out
hereinabove, by proposing a method for the automatic calibration of
a camshaft sensor that allows the value of a switching threshold to
be adjusted directly during a current revolution in accordance with
observations made in respect of the preceding teeth that have
already passed past the measurement cell during this same current
revolution.
[0016] To this end, and according to a first aspect, the present
invention proposes a method for the automatic calibration of a
camshaft sensor for a motor vehicle engine. The sensor comprises:
[0017] a toothed target comprising at least two teeth, [0018] a
measurement cell configured to supply a raw signal indicative of
the variations in a magnetic field which are induced by a rotation
of the target, and [0019] a processing module configured to supply,
from the raw signal, an output signal indicative of the moments at
which the teeth of the target pass past the cell.
[0020] The calibration method comprises, for each new revolution of
the target and for each tooth: [0021] determining a local minimum
of the raw signal as a space preceding said tooth passes past the
cell, [0022] determining a local maximum of the raw signal as said
tooth passes past the cell, [0023] calculating a switching
threshold for generating the output signal as a function of a local
minimum and of a local maximum which are determined for said tooth
in the preceding revolution of the target.
[0024] The calibration method is notable in that the switching
threshold is also calculated as a function of a corrective value
calculated as a function of a local maximum of the raw signal
determined during the passage of a preceding tooth past the cell
during the new revolution, and of a local maximum of the raw signal
determined during the passage of said preceding tooth past the cell
during a preceding revolution.
[0025] Thus, the calibration method makes it possible to determine,
for a new current revolution and for a given tooth of the target,
the value of a switching threshold not only as a function of a
local minimum and of a local maximum which are detected for said
tooth during the preceding revolution, but also as a function of a
corrective value calculated from a local maximum and/or a local
minimum of the raw signal during the passage of a preceding tooth
past the cell during the current revolution.
[0026] Such measures notably make it possible to adjust the value
of a switching threshold directly during the current revolution
when unexpected variations in the raw signal are observed for
preceding teeth which have already passed past the measurement cell
during the current revolution.
[0027] Thus, there is no need to wait for a complete calibration
revolution in order for these unexpected variations to be taken
into account. Such unexpected variations may, for example, arise as
a result of a change in the position of the target relative to the
measurement cell, for example following a knock or significant
vibrations (this is then referred to as a "jump in gap
distance").
[0028] In particular modes of implementation, the invention may
furthermore include one or more of the following features, taken
alone or in any technically feasible combination.
[0029] In particular implementations, said corrective value
corresponds to a difference between the local maximum determined
for said preceding tooth in the preceding revolution and the local
maximum determined for said preceding tooth in the new
revolution.
[0030] In particular implementations, said corrective value
corresponds to a difference between an amplitude of the raw signal
for said preceding tooth in the preceding revolution and an
amplitude of the raw signal for said preceding tooth in the new
revolution. What is meant by "an amplitude for a tooth in a given
revolution" is a difference between a local maximum and a local
minimum which are determined for that tooth during the revolution
concerned.
[0031] In particular implementations, said switching threshold is
calculated as a function of said corrective value only if the
corrective value is above or equal to a predetermined correction
threshold.
[0032] In particular implementations, a correction threshold is
defined for each tooth and for each revolution of the target, and a
correction threshold for said preceding tooth in the new revolution
of the target is defined by:
.DELTA.S.sub.k,N=(1-K).times.(M.sub.k,N-1-m.sub.k,N-1)
where: [0033] M.sub.k,N-1 corresponds to the value of the local
maximum determined for said preceding tooth in the preceding
revolution, [0034] m.sub.k,N-1 corresponds to the value of the
local minimum for said preceding tooth in the preceding revolution,
[0035] K is a predefined factor comprised between 0 and 1.
[0036] In particular implementations, said switching threshold is
calculated according to:
S.sub.j,N=K.times.(M.sub.j,N-1-m.sub.j,N-1-.DELTA..sub.k,N)+m.sub.j,N-1
where: [0037] Mj,.sub.N-1 corresponds to the value of the local
maximum determined for said tooth in the preceding revolution,
[0038] m.sub.j,N-1 corresponds to the value of the local minimum
determined for said tooth in the preceding revolution, [0039]
.DELTA..sub.k,N corresponds to the corrective value calculated for
said preceding tooth in the new revolution.
[0040] In particular embodiments, the factor K is comprised between
0.7 and 0.8.
[0041] According to a second aspect, the present invention relates
to a camshaft sensor for a motor vehicle engine. The sensor
comprises: [0042] a toothed target comprising at least two teeth,
[0043] a measurement cell configured to supply a raw signal
indicative of the variations in a magnetic field which are induced
by the rotation of the target, and [0044] a processing module
configured to supply, from said raw signal, an output signal
indicative of the moments at which the teeth of the target pass
past the cell.
[0045] The processing module is configured, for each new revolution
of the target and for each tooth, to: [0046] determine a local
minimum of the raw signal as a space preceding said tooth passes
past the cell, [0047] determine a local maximum of the raw signal
as said tooth passes past the cell, [0048] calculate a switching
threshold for generating the output signal as a function of a local
minimum and of a local maximum which are determined for said tooth
in the preceding revolution of the target.
[0049] The processing module is also configured to calculate the
switching threshold as a function of a corrective value calculated
as a function of a local maximum of the raw signal determined
during the passage of a preceding tooth past the cell during the
new revolution, and of a local maximum of the raw signal determined
during the passage of said preceding tooth past the cell during a
preceding revolution.
[0050] In particular embodiments, said corrective value corresponds
to a difference between the local maximum determined for said
preceding tooth in the preceding revolution and the local maximum
determined for said preceding tooth in the new revolution.
[0051] According to a third aspect, the present invention relates
to a motor vehicle comprising a camshaft sensor according to any
one of the above embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] The invention will be better understood upon reading the
following description, given by way of entirely non-limiting
example and with reference to FIGS. 1 to 5, in which:
[0053] FIG. 1: is a schematic depiction of a camshaft sensor,
[0054] FIGS. 2A-2B: are schematic depictions, for one tooth of the
target, of a raw signal indicative of the variations in magnetic
field which are induced by the rotation of the target of the
sensor, and of an associated output signal,
[0055] FIGS. 3A-3B: are schematic depictions of the raw signal and
of the corresponding output signal for one complete revolution of
the target,
[0056] FIG. 4: is a schematic depiction of the raw signal and of
the output signal for two consecutive revolutions of the target,
with a jump in gap distance on the second revolution,
[0057] FIG. 5: shows one particular implementation of the
automatic-calibration method according to the invention.
[0058] In these figures, references that are identical from one
figure to the next denote identical or analogous elements. For the
sake of clarity, the elements that are shown are not necessarily to
the same scale, unless stated otherwise.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0059] As indicated previously, the present invention seeks to
improve the precision of a motor vehicle engine camshaft
sensor.
[0060] FIG. 1 schematically depicts one example of a camshaft
sensor 10. This sensor 10 comprises a target 14, a magnetic-field
generator 11, a measurement cell 12, and an electronic
signal-processing module 13.
[0061] In one example considered and described entirely
nonlimitingly, the target 14 consists of a metal disk the periphery
of which is toothed, the magnetic-field generator 11 is a permanent
magnet, and the cell 12 for measuring the magnetic field is a
Hall-effect cell. As illustrated in FIG. 1, the measurement cell 12
is positioned at the level of the magnetic-field generator 11.
[0062] It should be noted that, according to another example, the
magnetic field measured by the measurement cell may be formed by
the target itself, which, as appropriate, is made of a magnetic
material. In such an instance, the target is "magnetically"
toothed, which means to say that the geometry of the periphery of
the target exhibits an alternation of North poles (equivalent to
the teeth in the example of FIG. 1) and south poles (equivalent to
the spaces in the example of FIG. 1).
[0063] The target 14 is fixed to a camshaft spindle in such a way
that the disk of the target 14 and the camshaft spindle are
coaxial. In other words, in an ideal situation, namely in the
absence of any lack of precision in the mounting of the target 14
on the camshaft, the axis of the camshaft spindle and the axis of
the target 14 coincide, and both pass through the center 15 of the
target 14.
[0064] The teeth D1, D2, D3 of the target 14 have respective
lengths I1, I2 and I3, and are separated from one another by spaces
of respective lengths s1, s2 and s3. In order to code for the
angular position of the target, the lengths I1, I2, I3, s1, s2, s3
of the teeth D1, D2, D3 and of the spaces are not all identical.
The teeth D1, D2, D3 generally all have the same height, but
deficiencies in the manufacturing of the target 14 may nevertheless
cause slightly different values to be observed for the respective
heights h1, h2, h3 of the teeth D1, D2, D3.
[0065] It should be noted that, in the example considered, the
target 14 comprises three teeth D1, D2, D3, but the invention also
applies to sensors 10 of which the target 14 comprises a different
number of teeth. In particular, the invention is applicable to a
target 14 comprising two or more teeth.
[0066] The rotation R of the target 14 and the successive passage
of the various teeth D1, D2, D3 past the magnetic-field generator
11 lead to variations in the magnetic field measured by the cell
12. In effect, the magnetic field varies as a function of the
magnitude of the gap e separating the magnetic-field generator 11
and the target 14.
[0067] The measurement cell 12 supplies the processing module 13
with a raw signal indicative of the strength of the magnetic field
measured. The processing module 13 is, for example, configured to
generate, from this raw signal, an output signal indicative of the
moments at which the various teeth D1, D2, D3 of the target 14 pass
past the measurement cell 12. The output signal may then make it
possible to recognize the moments at which the various teeth D1,
D2, D3 of the target 14 pass past the measurement cell 12 and,
ultimately, the angular position of the camshaft secured to the
target.
[0068] In order to do that, the processing module 13 comprises for
example one or more processors and storage means (electronic
memory) in which a computer program product is stored, in the form
of a set of program code instructions to be executed in order to
implement the various steps needed for generating said output
signal from the raw signal. Alternatively or in addition, the
processing module 13 comprises programmable logic circuits of FPGA,
PLD, etc. type, and/or specialized integrated circuits (ASIC),
and/or discrete electronic components, etc., suitable for
implementing these steps. In other words, the processing module 13
comprises means configured by software and/or by hardware to
implement the operations necessary for generating said output
signal from the raw signal.
[0069] FIG. 2A schematically depicts a portion of a raw signal 20
indicative of the variations in the magnetic field measured by the
cell 12. The strength B of the magnetic field is represented on the
ordinate axis while the time t is represented on the abscissa
axis.
[0070] The portion of the raw signal 20 depicted in FIG. 2A
corresponds for example to a passage of a tooth D.sub.j of index j
past the measurement cell 12 during a revolution N of the target
14.
[0071] In the example considered, the target 14 comprises three
teeth D1, D2, D3, and the index j therefore varies between 1 and 3.
The number N corresponds for example to the number of complete
revolutions performed by the target 14 since an initialization of
the sensor 10 corresponding, for example, to an application of
power to the processing module 13.
[0072] The raw signal 20 thus exhibits a high part with a rising
front 21 corresponding to the start of the passage of the tooth
D.sub.j past the cell 12, and a falling front 22 corresponding to
the end of the passage of the tooth D.sub.j past the cell 12. The
rising front corresponds to a sharp increase in the magnetic field
strength caused by the sharp decrease in the magnitude of the gap e
as the tooth D.sub.j begins to pass past the cell 12 (the
transition from a space to a tooth). The falling front corresponds
to a sharp decrease in the magnetic field strength caused by the
sharp increase in the magnitude of the gap e as the tooth D.sub.j
completes its passage past the cell 12 (the transition from a tooth
to a space). Between the rising front 21 and the falling front 22,
the signal 20 adopts a value that is more or less constant assuming
that the magnitude of the gap remains substantially identical
throughout the time taken for the tooth D.sub.j to pass past the
cell 12.
[0073] FIG. 2B schematically depicts a portion of an output signal
30 generated by the processing module 13 from the raw signal
20.
[0074] This output signal 30 is for example an electrical signal
adopting a positive value (for example 5V) when a tooth D1, D2, D3
is facing the cell 12, and a zero value (0V) when a space is facing
the cell 12. The electrical voltage V of the output signal 30 is
represented on the ordinate axis and the time t is represented on
the abscissa axis.
[0075] The output signal 30 thus comprises a succession of square
waveforms. The high part of each square form corresponds to the
passage of a tooth D1, D2, D3 of the target 14 past the measurement
cell 12. The high part of each square form comprises a rising front
31 and a falling front 32 corresponding more or less to the passing
of the mechanical fronts of a tooth D1, D2, D3 past the measurement
cell. The portion of the raw signal 30 depicted in FIG. 2B
corresponds for example to a passage of the tooth D.sub.j past the
cell 12.
[0076] In general, the moment of each rising front 31 and falling
front 32 of the output signal 30 (namely each transition of the
electrical signal) is determined, for the tooth D.sub.j in the
revolution N of the target 14, from a switching threshold S.sub.j,N
that is predetermined for the raw signal 20. In other words, the
output signal 30 exhibits a rising front 31 when the raw signal 20
passes above the switching threshold S.sub.j,N, and the output
signal 30 exhibits a falling front 32 when the raw signal 20 passes
below the switching threshold S.sub.j,N.
[0077] On each revolution N of the target 14, the processing module
13 is able to determine and store in memory a local maximum
M.sub.j,N and a local minimum m.sub.j,N which are observed on the
raw signal 20 for the tooth D.sub.j. From that it is then possible
to deduce an amplitude A.sub.j,N of the raw signal 20 for the tooth
D.sub.j in the revolution N, as being equal to the difference
between M.sub.j,N and m.sub.j,N.
[0078] The switching threshold S.sub.j,N is for example calculated
from a percentage of the amplitude A.sub.j,N-of the raw signal 20
for the tooth D.sub.j during the preceding revolution N-1 of the
target 14. The threshold S.sub.j,N conventionally corresponds to a
value chosen from a range comprised between 70% and 80% of the
amplitude A.sub.j,N-1, preferably around 75%. In other words, for a
factor K comprised between 0 and 1, generally comprised between 0.7
and 0.8, and preferentially amounting to 0.75, the threshold
S.sub.j,N is conventionally defined by:
S.sub.j,N=m.sub.j,N-1+A.sub.j,N-1.times.K (1)
[0079] The remainder of the description, considers, by way of
nonlimiting example, the case in which K=75%.
[0080] It is known practice, as illustrated by way of example in
part a) of FIG. 2, to detect a local minimum m.sub.j,N (or,
respectively, the local maximum M.sub.j,N) for the tooth D.sub.j in
the revolution N of the target 14 when the raw signal 20 varies by
a value that is greater (in terms of absolute value) than a
predefined constant C after its gradient has become positive (or,
respectively, negative).
[0081] This can be repeated for each revolution of the target 14
and for each tooth D1, D2, D3 of the target 14 so as to obtain, for
a revolution N of the target 14, a value S.sub.j,N for the
switching threshold to be used. This may be the one same switching
threshold S.sub.N to be used for all the teeth D1, D2, D3 (the
value of this threshold being calculated for example as a function
of a mean, minimum or maximum value of the local minima m.sub.j,N-1
and/or of the local maxima M.sub.j,N-1 observed for the teeth D1,
D2, D3 in the preceding revolution N-1), or else it may be a
switching threshold S.sub.j,N that is different for each tooth
D.sub.j (the value of this threshold being calculated for example
as a function of the local maximum M.sub.j,N-1 and of the local
minimum m.sub.j,N-1 which are observed for the tooth D.sub.j in the
preceding revolution N-1).
[0082] It may be advantageous to determine a switching threshold
S.sub.j,N that is different for each tooth D.sub.j, particularly if
deficiencies with the geometry of the target 14 or a phenomenon of
runout (misalignment of the axis of the target 14 with that of the
camshaft) are leading to local maxima values that differ for the
various teeth of the target.
[0083] It should also be noted that it is conceivable for the
switching threshold S.sub.j,N not to be recalculated for each new
revolution of the target but, for example, updated less frequently
than the frequency of rotation of the target 14, for example each
time the target 14 has performed a predetermined number of
revolutions. Nevertheless, it is advantageous for the switching
threshold S.sub.j,N to be recalculated regularly, and preferably
for each revolution of the target 14, in order to compensate for
variations in the raw signal 20 during the course of operation of
the sensor (for example as a result of variations in
temperature).
[0084] FIG. 3A schematically depicts the way in which the raw
signal 20 evolves during a revolution N of the target 14. Three
high parts succeed one another, these corresponding respectively to
the passage of the three teeth D1, D2, D3 of the target 14 past the
measurement cell 12. FIG. 3B for its part schematically depicts the
evolution of the output signal 30 generated from the raw signal 20
and from the thresholds S.sub.1,N, S.sub.2,N, S.sub.3,N calculated
for the teeth D1, D2, D3 according to one of the solutions
described earlier.
[0085] It proves to be the case that, in certain operating
scenarios, these solutions do not allow the camshaft sensor 10 to
achieve sufficient precision. The inventor has notably discovered
that a change in the position of the target 14 relative to the
measurement cell 12 can occur, for example as the result of
vibrations or knocks. That has the effect of moving the teeth D1,
D2, D3 closer to or further away from the measurement cell 12,
namely of reducing or increasing the magnitude of the gap e
separating a tooth D1, D2, D3 from the target 14 as it passes past
the measurement cell 12. A variation in the magnitude of the gap e
leads to a variation in the magnetic field measured by the
measurement cell 12.
[0086] In the remainder of the description, such a sudden variation
in the magnitude of the gap separating a tooth D1, D2, D3 from the
measurement cell 12 is referred to as a "jump in gp distance".
[0087] FIG. 4 schematically depicts the effect of a jump in gap
distance on the raw signal 20.
[0088] FIG. 4 depicts the way in which the raw signal 20 evolves
during two revolutions of the target 14. From left to right, the
first three high parts of the raw signal 20 correspond respectively
to the passage of the teeth D1, D2, D3 past the measurement cell 12
during one revolution N-1, then the next three high parts
correspond respectively to the passage of the teeth D1, D2, D3 past
the measurement cell 12 during the next revolution N.
[0089] In the example illustrated in FIG. 4, a jump in gap distance
occurs as the tooth D1 passes past the measurement cell 12 during
the revolution N. It is notably possible to observe a sudden drop
in the raw signal 20 at a moment corresponding more or less to the
middle of the passage of the tooth D1 past the measurement cell 12.
This sudden drop in the raw signal 20 in the middle of the passage
of the tooth D1 past the measurement cell 12 is caused by a jump in
gap distance which moves the target 14 (and therefore the teeth D1,
D2, D3) away from the measurement cell 12.
[0090] While the switching threshold S.sub.1,N calculated for
example in accordance with formula (1) remains suitable for
determining the moment of the rising front 31 of the output signal
30 representing the start of the passage of the tooth D1 past the
measurement cell 12 in the revolution N, it is, by contrast,
entirely ill-suited to determining the moment of the falling front
32 of the output signal 30 indicative of the end of the passage of
the tooth D1 past the measurement cell 12 in the revolution N.
Specifically, because of the jump in gap distance that has occurred
in the middle of the passage of the tooth D1 past the measurement
cell 12, the switching threshold S.sub.1,N now no longer
corresponds to 75% of the amplitude of the raw signal 20 observed
at the end of the passage of the tooth D1 past the measurement cell
12. Therefore, the moment of the falling front of the output signal
30 corresponding to the passage of the mechanical front of the
tooth D1 past the measurement cell 12 at the end of the passage of
the tooth D1 is detected too early, and the precision of the sensor
10 is impaired. If nothing is done about this, similar precision
errors will impact on the following teeth D2 and D3 during
revolution N.
[0091] The remainder of the description concerns itself with
describing a method for calibrating the camshaft sensor 10 that
makes it possible to correct the switching thresholds S.sub.2,N and
S.sub.3,N for the teeth D2 and D3 which pass past the measurement
cell 12 after the tooth D1 during revolution N
[0092] Thus, where conventionally it was necessary to wait for the
target 14 to perform at least one full revolution after a jump in
gap distance in order to find switching-threshold values suited to
the actual values of the raw signal 20, the method according to the
invention allows the switching thresholds to be used for the teeth
that follow the tooth for which the jump in gap distance was
detected to be corrected directly during the current revolution.
The precision of the sensor 10 is thus improved.
[0093] It should be noted that, in the example considered and
illustrated in FIG. 4, the jump in gap distance has a negligible
impact on the values adopted by the raw signal 20 when the spaces
pass past the measurement cell 12. In particular, the values of the
local minima m.sub.j,N observed during revolution N are
substantially identical to those of the local minima m.sub.j,N-1
observed during revolution N-1. Indeed experience shows that a jump
in gap distance does not generally change the magnitude of the
magnetic field measured by the cell 12 in the presence of a space,
as in any case at that moment the magnetic field is at a low value
equivalent to that which would be measured in the complete absence
of a metallic target 14.
[0094] Before applying a switching threshold S.sub.j,N as
calculated according to formula (1) for a tooth D.sub.j on a
revolution N of the target 14, the method according to the
invention also determines, for said tooth D.sub.j, whether a
corrective value .DELTA..sub.k,N needs to be applied to said
switching threshold S.sub.j,N. The corrective value .DELTA..sub.k,N
is calculated as a function of observations made on a preceding
tooth D.sub.k during the same revolution (0<k<j).
[0095] For example, if it is found, for a tooth D.sub.k that the
local maximum M.sub.k,N observed for the tooth D.sub.k during
revolution N differs from the local maximum M.sub.k,N-1 observed
for the tooth D.sub.k during the preceding revolution N-1, then a
corrective value .DELTA..sub.k,N equal to this difference may be
applied to the switching threshold S.sub.j,N.
[0096] In particular implementations, the corrective value
.DELTA..sub.k,N is taken into consideration in calculating the
switching threshold S.sub.j,N only if it is higher than or equal to
a predetermined correction threshold .DELTA.S, this being so as to
avoid unwarranted corrections caused by minor variations in the raw
signal 20 from one revolution of the target 14 to another.
[0097] FIG. 5: shows the main steps of one particular
implementation of the method according to the invention for
automatically calibrating the sensor 10. Steps E1 to E9 illustrated
in FIG. 5 are implemented during one revolution N of the target 14
and are repeated for each new revolution. In the example
considered, the method is implemented by the processing module 13
of the sensor 10.
[0098] In an initialization step E1 for a revolution of rank N, a
counter j indicating the index for the next tooth D.sub.j that is
to pass past the measurement cell 12 is initialized to the value 1
(the teeth are numbered from 1 to n, n being the number of teeth of
the target 14).
[0099] In a step E2, a corrective value A is calculated as a
function of that which has been observed for the tooth D.sub.k
situated just before the tooth D.sub.j on the target 14. If the
index j is equal to 1, it is appropriate to pay attention to that
which was observed for the last tooth D.sub.n in the previous
revolution N-1. The corrective value is then denoted
.DELTA..sub.n,N-1. If the index j is strictly greater than 1, it is
appropriate to pay attention to that which was observed for the
preceding tooth D.sub.j-1 in revolution N. The corrective value is
then denoted .DELTA..sub.j-1,N.
[0100] In the particular implementation being considered at the
present moment, a corrective value .DELTA..sub.k,N for a tooth
D.sub.k in revolution N corresponds to the difference between a
local maximum M.sub.k,N-1 observed on the raw signal 20 as the
tooth D.sub.k passes past the measurement cell 12 in revolution N-1
and a local maximum M.sub.k,N observed on the raw signal 20 when
the tooth D.sub.k passes past the measurement cell 12 in revolution
N:
.DELTA..sub.k,N=M.sub.k,N-1-M.sub.k,N (2)
[0101] A step E3 checks whether the corrective value .DELTA.
calculated in step E2 is above a predetermined correction threshold
.DELTA.S.
[0102] If the collective value .DELTA. is above the correction
threshold .DELTA.S, then in a step E4, the local maxima detected
and stored during the previous revolution N-1 for the teeth D.sub.i
which have not yet passed past the measurement cell 12 in the
revolution N are adjusted with the corrective value .DELTA.:
For j.ltoreq.i.ltoreq.n:M.sub.i,N-1M.sub.i,N-1-.DELTA.
[0103] In a step E5, the switching threshold S.sub.j,N to be used
for the tooth D.sub.j in revolution N can then be calculated using
formula (1), it being understood that the corrective value .DELTA.
is incorporated into the calculation of S.sub.j,N through the
adjustment of M.sub.j,N-1 which has been performed in step E4.
[0104] In a step E6, the local minimum m.sub.j,N and the local
maximum M.sub.j,N are detected then stored in memory in order to be
used later.
[0105] A step E7 checks whether the tooth D.sub.j corresponds to
the last tooth to pass past the measurement cell 12 for the
revolution N. If it is, the revolution counter is incremented to
the value N+1 (step E8) and the method resumes from step E1. If
not, the tooth counter is incremented to the value j+1 (step E9)
and the method resumes from step E2.
[0106] By applying the steps of the method described with reference
to FIG. 5 to the example illustrated in FIG. 4, it will be
appreciated that: [0107] before the tooth D2 (j=2) passes past the
measurement cell 12 in revolution N of the target 14, a corrective
value .DELTA..sub.1,N is calculated in step E2 as being the
difference between the local maximum M.sub.1,N-1 observed for the
tooth D1 in revolution N-1 and the local maximum M.sub.1,N for the
tooth D1 in revolution N, [0108] this corrective value
.DELTA..sub.1,N (which in the example being considered corresponds
to a jump in gap distance) is higher than or equal to a
predetermined correction threshold .DELTA.S, and therefore the
local maxima M.sub.2,N-1 and M.sub.3,N-1 are adjusted in step E4,
[0109] the threshold S.sub.2,N calculated in step E5 is thus
corrected with respect to the jump in gap distance by virtue of the
adjustment of the local maximum M.sub.2,N-1 using the corrective
value .DELTA..sub.1,N, [0110] for tooth D3 (j=3), the corrective
value .DELTA..sub.2,N is below the correction threshold AS insofar
as the local maximum M.sub.3,N-1 has already been adjusted
beforehand using the corrective value .DELTA..sub.1,N in order to
correct for the effects of the jump in gap distance observed for
tooth D1. The threshold S.sub.3,N is thus also corrected by the
corrective value .DELTA..sub.1,N.
[0111] The calibration method has thus made it possible to correct
the switching thresholds S.sub.2,N and S.sub.3,N for teeth D2 and
D3 directly during revolution N, and there is no need to wait for a
full revolution of the target 14 in order for the effects of the
jump in gap distance to be corrected.
[0112] It should be noted that there are various conceivable
methods for defining the corrective value .DELTA..sub.k,N.
[0113] For example, the corrective value .DELTA..sub.k,N may
correspond to the difference between an amplitude A.sub.k,N-1
observed on the raw signal 20 as the tooth D.sub.k passes past the
measurement cell 12 in revolution N-1 and an amplitude A.sub.k,N
observed on the raw signal 20 when the tooth D.sub.k passes past
the measurement cell 12 in revolution N:
.DELTA..sub.k,N=A.sub.k,N-1-A.sub.k,N=(M.sub.k,N-1m.sub.k,N-1)-(M.sub.k,-
N-m.sub.k,N) (3)
[0114] Calculating a corrective value as a function of a difference
in amplitude rather than a difference in local maximum may prove
beneficial if the jump in gap distance has an impact not only on
the local maximum but also on the local minimum.
[0115] According to another example, the corrective value
.DELTA..sub.k,N may be determined by comparing two successive
teeth, rather than by comparing the same tooth on two consecutive
revolutions.
[0116] It is however advantageous to determine the corrective value
.DELTA..sub.k,N by comparing the same tooth on two consecutive
revolutions, notably if deficiencies with the geometry of the
target 14 or a phenomenon of runout lead to different local maxima
values for the various teeth of the target (in which case a
comparison between two successive teeth is not necessarily
relevant).
[0117] According to yet another example, the corrective value
.DELTA..sub.k,N may correspond to a difference between two local
minima.
[0118] The particular choice made of a method for determining the
corrective value .DELTA..sub.k,N merely represents a variant of the
invention.
[0119] Furthermore, it is equally possible to define a different
correction threshold for each tooth of the target 14. For example,
a correction threshold .DELTA.S.sub.k,N may thus be defined for
each tooth D.sub.k and for each revolution N of the target, such
that:
.DELTA.S.sub.k,N=(1-K).times.(M.sub.k,N-1-m.sub.k,N-1) (4)
[0120] Such a choice of correction threshold makes it possible to
ensure that the detection of a tooth is not missed, while at the
same time limiting instances in which a correction is made.
Specifically, a higher correction threshold could lead to instances
in which an uncorrected switching threshold is higher than a local
maximum of the raw signal 20, and this would lead to a missed
detection of a tooth. If, on the other hand, the correction
threshold is too low, an excessive number of corrections could lead
to suboptimal operation of the sensor 10.
[0121] It should be noted that steps E1 to E9 described with
reference to FIG. 5 are valid for N>2, namely starting from a
third revolution of the target 14 since the initialization of the
sensor 10 upon application of power thereto. Specifically, during
the first revolution of the target 14 (N=1), there are no local
maxima and minima values stored in memory for a preceding
revolution. Thus, during the second revolution of the target 14
(N=2), it is not possible to determine the corrective value
.DELTA..sub.n,1 in order to potentially correct the threshold
S.sub.1,2. For the first two revolutions of the target 14, a set
default value is for example used for the switching thresholds
while awaiting the obtaining of the local maxima and minima values
that will allow said switching thresholds to be calibrated more
precisely.
[0122] The description above clearly illustrates that, through its
various features and the advantages thereof, the present invention
achieves the set aims. In particular, the calibration method
according to the invention makes it possible to determine with
greater precision the moments of a rising front 31 and of a falling
front 32 of the output signal 30 corresponding respectively to the
moments marking the beginning and end of the passage of the
mechanical fronts of a tooth D1, D2, D3 as said tooth D1, D2, D3
passes past the measurement cell 12. In the event of a jump in gap
distance, the calibration method is able to react directly and
adjust the switching thresholds without the need to wait for a full
calibration revolution in order for the effect of the jump in gap
distance to be able to be accounted for.
* * * * *